The conventional image-forming process of silver halide photography includes imagewise exposure of a color photographic silver halide recording material to actinic radiation (such as visible light), and the eventual manifestation of a useable image by wet photochemical processing of that exposed material. A fundamental step of photochemical processing is the treatment of the material with one or more developing agents to reduce silver halide to silver metal. A useful color image consists of one or more images in organic dyes produced from an oxidized developing agent formed wherever silver halide is reduced to metallic silver.
To obtain useful color images, it is usually necessary to remove all of the silver from the photographic element after color development. This is sometimes known as "desilvering". Removal of silver is generally accomplished by oxidizing the metallic silver, and then dissolving it and undeveloped silver halide with a "solvent" or fixing, agent in what is known as a fixing step. Oxidation is achieved with an oxidizing agent, commonly known as a bleaching agent.
Color photographic elements can be designed to provide either color negative or color positive images. For example, color negative images can be produced by imaging and appropriate color processing of color negative films. The typical commercial processing methods for such films generally include color development, bleaching, fixing and final rinsing or stabilizing steps.
Another commercially important process intended for providing positive color images, can include the following sequence of processing steps: first (or black-and-white) development, washing, reversal reexposure, color development, bleaching, fixing, washing and/or stabilizing. Another useful process has the same steps, but stabilizing is carried out between color development and bleaching. Such conventional steps are described, for example, in U.S. Pat. No. 4,921,779 (Cullinan et al), U.S. Pat. No. 4,975,356 (Cullinan et al), U.S. Pat. No. 5,037,725 (Cullinan et al), U.S. Pat. No. 5,523,195 (Darmon et al) and U.S. Pat. No. 5,552,264 (Cullinan et al) for the processing of color reversal films.
The most common bleaching agents for color photographic processing are complexes of ferric ion and various organic chelating ligands (such as aminopolycarboxylic acids), of which there are hundreds of possibilities, all with varying bleaching activities and biodegradability. Common organic chelating ligands used as part of bleaching agents for color film processing include ethylenediamine-tetraacetic acid (EDTA), 1,3-propylenediaminetetraacetic acid (PDTA) and nitrilotriacetic acid (NTA).
U.S. Pat. No. 4,294,914 (Fyson) describes bleaching and bleach-fixing compositions and a processing method using a ferric complex of one of several alkyliminodiacetic acids, which are known to be more biodegradable than other common organic chelating ligands such as EDTA. Other bleaching agents using similar organic chelating ligands are described in U.S. Pat. No. 5,061,608 (Foster et al) in which the bleaching agent is advantageously combined with specific aliphatic carboxylic acids to reduce dye stains. U.S. Pat. No. 5,334,491 (Foster et al) also describes the use of similar biodegradable bleaching agents in combination with specific levels of bromide ion.
The use of biodegradable bleaching agents is becoming more important as governmental regulators and photochemical users become more aware of the need to reduce the impact on the environment. There is considerable literature relating to such bleaching agents, including ferric complexes of methyliminodiacetic acid (MIDA) and similar compounds. Bleaching compositions containing iron complexes of MIDA and other biodegradable ligands have been used successfully for processing color negative films.
JP Kokai 5-26542 describes a bleaching solution containing an iron chelate and 2-carboxypyridine. Other literature describes the use of uncomplexed heterocyclic compounds such as pyridine-2,6-dicarboxylic acid in processing solutions. U.S. Pat. No. 5,536,625 (Buchanan et al) describes the use of ferric ion complexes of such heterocyclic compounds as peracid bleaching accelerators. Ternary complexes comprising the heterocyclic compounds are similarly described in U.S. Pat. No. 5,521,056 (Buchanan et al).
The same ternary complexes are used as bleaching agents in U.S. Pat. No. 5,582,958 (Buchanan et al) wherein one of the ligands complexed to ferric ion is for example pyridinecarboxylic acid (PCA) or 2,6-pyridinedicarboxylic acid (PDCA). Such chelating ligands are also biodegradable, making them even more attractive for photographic processing compositions.
It has been observed, however, that a possible problem arising with the use of such biodegradable chelating ligands is that precipitates tend to form in bleaching compositions containing such ligands after storage for several days. In other words, the shelf stability of the bleaching compositions is too short. Precipitate formation is undesirable for a number of reasons including that it can cause deposits on processing equipment, it changes the concentration of the remaining components in the solution, and it can require filtration steps.
Thus, there is a need to provide bleaching compositions containing biodegradable ligands such as pyridinecarboxylic acids that have increased shelf stability.